Programmable logic controllers are used throughout industry to control and monitor a wide range of machines and other movable components and systems. Conventional PLC systems utilize a plurality of modules, e.g. input/output modules, that are mechanically mounted in a rack system and electronically connected along a back plane. The individual modules can be interchanged or replaced by disconnecting them from the rack and/or back plane. It would be advantageous to eliminate the mounting components, e.g. rack and back plane, and to provide modules that were in an interlocking, modular form.
At least one attempt has been made to construct modules that may be interconnected to each other without the use of a rack or back plane. In this particular embodiment, each module includes a stationary plug and a stationary plug receptacle designed for mating engagement with the stationary plug of a next adjacent module. Additionally, each module includes a housing having a plurality of tongs on one side and a plurality of openings on the opposite side to receive the extended tongs of the next adjacent module. Each module also includes a mounting mechanism for mounting along a DIN rail.
Thus, an individual module can be attached to an adjacent module by moving the individual module along the DIN rail until the tongs and plug connector engage the openings and plug connector of the next adjacent module. Once engaged, locking mechanisms are used to hold the module prongs within the corresponding openings of the next adjacent module.
This configuration does not permit mechanical interlocking of adjacent modules independent of forming an electrical connection between modules. Additionally, the design does not permit the removal of an individual module that is sandwiched between adjacent modules without upsetting the mounting of additional I/O modules. The mechanical interlocking system requires that a module be moved laterally sufficiently far to clear its engagement prongs from the openings of the next adjacent module. To permit this lateral movement, all of the modules positioned on one side of the subject module must be moved to provide sufficient clearance for removal and/or insertion of the subject module. Once the subject module is engaged, the string of modules moved out of the way must be reengaged.
It would be advantageous to have a compact, rackless, modular PLC system utilizing modules that could be selectively, mechanically interlocked and yet readily inserted or removed regardless of whether the module was disposed between adjacent modules.